1. Field of the Invention
The present invention relates to a supported bimetallic catalyst, its preparation and its use in the production of homo- and copolymers of ethylene (polyethylene resins) with a bimodal molecular weight distribution (MWD) in a single reactor. The present invention also relates to polyethylene resins produced with a catalyst that includes the supported bimetallic catalyst.
2. Background
Swell characteristics play an important role in determining the processability of high density polyethylene (HDPE) blow molding resins, such as those used for the manufacture of bottles and similar articles. More particularly, when a polymer is melted and then forced through a small opening (orifice) the polymer may swell (expand) to a diameter larger than the orifice. This phenomenon is commonly referred to as “polymer swell”. “Intrinsic swell” is the polymer swell as measured from a polymer sample directly after manufacture. To obtain the intrinsic swell of a polymer sample, the polymer sample must be completely stabilized with an additive package (i.e., compounds that prevent any polymer degradation when the polymer is melted) during the time the swell measurement is being made.
It is known that HDPE blow molding resins are produced in the gas phase and in a single reactor by means of traditional chromium-based catalysts. However, the intrinsic swell of the resulting resins often is too high for commercial applications; i.e., so high that it contributes to, for example, unacceptably high bottle weight, poor bottle handle and fixtures formation, and excessive flash. One option for lowering the swell of such resins is to degrade them in the presence of air along with high stresses and temperature. However, after the swell has been lowered due to polymer degradation, the resin needs to be stabilized with antioxidants to prevent further polymer degradation. Another option for lowering the swell is the use of peroxides having high decomposition temperatures to produce controlled degradation. A potential disadvantage of this technique is that it requires well-controlled downstream processing of the resin, which requires maintenance and adds to the cost of the resin. Both of these controlled degradation techniques can lead to contamination of the resin and/or color and odor problems in the resin. Moreover, these polymer degradation processes slow down the manufacturing rates (polymer output/time unit) and may be difficult to control. On the other hand, if the swell of a particular polymer is too low for commercial applications, nothing can be done to increase swell to the necessary value.
Generally, high performance blow molding resins have a bimodal molecular weight distribution (MWD). As used herein, “resin having a bimodal MWD” means that the resin comprises at least two polymer components, one of the at least two components having a higher average molecular weight (hereinafter sometimes referred to as the “HMW polymer component”) than another of the at least two components (hereinafter sometimes referred to as the “LMW polymer component”). Resins with a bimodal MWD can be produced in a single reactor using the technology disclosed in, for example, U.S. Pat. No. 5,539,076, discussed below, or by the use of a series of reactors or reaction steps. For example, bimodal MWD polyethylene resins can be produced in tandem slurry processes, but often suffer from low intrinsic swell. Low intrinsic swell leads to problems with webbing in the bottle, poor formation of handles and bottle fixtures, and bottle extrusion problems.
U.S. Pat. No. 5,032,562 to Lo et al. discloses a supported olefin polymerization catalyst composition comprising a precursor and a catalyst activator. The catalyst is used in the presence of small amounts of hydrogen to produce polyolefins having a multimodal MWD in a single reactor. The catalyst comprises a dialkylmagnesium component, a zirconocene and a non-metallocene titanium and/or vanadium compound. It is mentioned that the precursor may optionally also include an organic compound, suitable examples thereof being alcohols, ketones, esters, acids or organic silicates. Alcohols, such as 1-butanol, are stated to be the preferred organic compounds.
U.S. Pat. No. 5,539,076 to Nowlin et al. discloses resins which are in situ catalytically produced polyethylene resin blends of a broad bimodal MWD that can be processed into films on existing equipment and exhibit good processability in blown film production and reduced tendency towards die-lip buildup and smoking in on-line operations. The preferred catalyst for producing these resins comprises a catalyst including a support treated with a dialkylmagnesium compound, an aluminoxane, at least one metallocene and a non-metallocene transition metal source as well as an alkylaluminum compound, e.g., trimethylaluminum (TMA), as cocatalyst.
U.S. Pat. No. 5,614,456 to Mink et al. is directed to an activated catalyst composition for producing bimodal MWD high density and linear low density polyethylene resins, which activated catalyst does not require an alkylaluminum cocatalyst. A preferred catalyst comprises, as support, silica impregnated with a dialkylmagnesium compound and an organic alcohol reagent, e.g., butanol. The support is contacted with at least two transition metal compounds, at least one of which is a metallocene, e.g., zirconocene, and, as activator, aluminoxane, either alone or admixed with the metallocene compound.
U.S. Pat. No. 5,260,245 to Mink et al. describes a catalyst for producing higher flow index linear low density polyethylene with relatively narrower molecular weight distributions. The catalyst is formed by treating silica having reactive OH groups with a dialkylmagnesium compound and a carbonyl-containing compound to form an intermediate which is subsequently treated with a transition metal compound to form a catalyst precursor. The catalyst precursor is activated with triethylaluminum.
PCT publication WO 97/35891 is directed to a process of forming a bimetallic catalyst composition comprising a cocatalyst and a catalyst precursor. The precursor comprises at least two transition metals; a metallocene complex is a source of one of said two transition metals. The precursor is produced in a single-pot process by contacting a porous carrier, in sequence, with a dialkylmagnesium compound, an aliphatic alcohol, a non-metallocene transition metal compound, a contact product of a metallocene complex and a trialkylaluminum compound, and methylalumoxane.